JP3649175B2 - Pretreatment method for long-time continuous casting - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a pretreatment method for long-time continuous casting, and in particular, is a novel proposal for a secondary refining method suitable for long-time stable casting.
[0002]
[Prior art]
In the case of special steels such as stainless steel and high alloy steel, low-speed casting is often performed from the viewpoint of maintaining quality, and there is a problem that a long time is required for continuous casting. That is, in recent years, demands for functions and quality of steel materials have increased, and accordingly, improvement in slab quality at the steel making stage has been demanded. In order to meet these demands, there has been proposed a continuous casting method for reducing the casting speed for the purpose of suppressing the intrusion of inclusions during continuous casting and reducing cross-sectional defects such as center segregation. However, if the casting speed is reduced and the casting time is extended, the molten steel temperature of the ladle remaining steel gradually decreases, and as a result, the degree of superheat of the molten steel also decreases.
[0003]
In general, the degree of superheat of molten steel during continuous casting (difference between the temperature of the tundish molten steel and the solidification temperature) is preferably high in order to promote the flotation of inclusions. May cause operational troubles such as breakout. On the other hand, when an equiaxed crystal is desired as the solidification structure, it is desirable that the degree of superheating of the molten steel is low, and there is an advantage that the reduction of the molten steel temperature can be reduced in terms of refining costs. However, if it is too low, there is a problem that casting cannot be continued or the surface defect of the slab is increased due to hindering the floating separation of inclusions. It is.
[0004]
For such problems, conventionally, when casting a steel type that is forced to perform low-speed casting as described above, measures such as a method of increasing the molten steel temperature at the end of refining and ensuring the molten steel superheat degree at the end of casting are taken. Has been taken. However, in such a method, the difference in the degree of superheated molten steel becomes large between the initial stage and the final stage of casting, which often causes slab quality variations and operational troubles.
The solution is to increase the temperature of the molten steel by using electric energy such as induction heating or plasma torch in the tundish when the casting time is long or when it is necessary to control the degree of superheat of the molten steel. Means for compensating for the temperature drop of the molten steel at the end of casting have been studied (for example, Japanese Patent Laid-Open No. 1-237064).
[0005]
[Problems to be solved by the invention]
The above-mentioned prior art for continuous casting for a long period of time is necessary when tundish modification is required, special equipment such as a tundish molten steel heating device is required, or when the superheat degree of the molten steel is increased. There was a problem that the lining life of the pan and tundish was shortened and the cost was increased. In addition, these methods have a problem in that variations in slab quality inevitably occur due to a decrease in thermal efficiency and a significant fluctuation (drop) in molten steel temperature from the beginning to the end of casting.
[0006]
Accordingly, the object of the present invention is to provide a stable and continuous operation over the entire casting period without requiring any special heating device or high degree of superheated molten steel, without slab quality variations and operational troubles. The technology that can be cast is established and proposed.
[0007]
As a result of diligent research toward the realization of the above-mentioned object, the inventors have found that the invention according to the following summary configuration can advantageously solve the above-described problems of the prior art, and arrived at the present invention. did. That is, according to the present invention, when performing continuous casting for a long time of about 70 minutes / ch or more for continuous casting of a ladle 1 charge, in the secondary refining stage in the ladle prior to this continuous casting. The treatment is performed for about 40 minutes or more when the temperature of the refractory inside the ladle is almost stabilized.
[0009]
Moreover, it is preferable that the time for stabilizing the temperature of the ladle inner wall refractory, that is, the time for stabilizing the amount of heat released from the ladle is about 40 minutes or more.
[0010]
DETAILED DESCRIPTION OF THE INVENTION
The inventors have studied the solution to the above-mentioned problems that the conventional technology has, and for the temperature drop at the end of casting during continuous casting for a long time, It has been found that the heat content is greatly affected by the secondary refining conditions, which is the pre-treatment of continuous casting.
In general, it is known in the field of steelmaking that the heat content of the ladle affects the temperature drop of the molten steel in the ladle. In order to suppress the temperature drop, the ladle before receiving the steel is known. A method of increasing the heat content is employed by heating the hot pot with a combustion burner or the like, or reducing the number of operating ladles and increasing the usage ratio. In this regard, the inventors have also taken measures such as sufficiently performing preheating with a burner or increasing the heat content in advance by using a general-purpose steel grade for casting, especially in cases where casting for a long time is forced. However, in some cases, the temperature of the molten steel dropped too much at the end of casting, and it was impossible to continue casting.
[0011]
Therefore, the inventors examined the secondary refining time to be performed prior to continuous casting in research on factors affecting the temperature drop at the end of casting. As a result, it was found that this secondary refining time strongly influences the decrease in molten steel temperature at the end of casting. The time of secondary refining is generally agitation to ensure the temperature and composition uniformity of the molten steel, floating separation of non-metallic inclusions floating in the molten steel, and gas components such as C, N, H, O in the molten steel It is determined by the time required to bring the composition, temperature, cleanliness, etc. of the molten steel due to vacuum removal or temperature rise into the expected range. Therefore, the secondary refining time varies greatly depending on the specifications required for the molten steel and the conditions of the molten steel used for the secondary refining treatment, and the secondary refining time can be reduced by various improvements to reduce refining costs and refractory costs. Has also been shortened.
However, from the viewpoint of the thermal energy possessed by the refractory (lining) of the ladle, when observing the secondary refining time, as shown in Fig. 1, the amount of heat supplied from the molten steel to the ladle depends on the heating of the burner. It is overwhelmingly larger than the amount of heat that can be supplied. When considering only when using the ladle, the amount of heat that is sufficient to compensate for the variation in the heat content due to the difference in the number of times this ladle has been used continuously. It turned out that it was the quantity which can be supplied.
[0012]
Therefore, the inventors tried to change the heat supply amount from the molten steel to the ladle according to the secondary refining time. That is, an experiment to intentionally shorten or extend the secondary refining time was carried out, and the change in molten steel temperature at the end of casting during long casting was investigated. As a result, in the charge with a short secondary refining time, the temperature of the molten steel decreased significantly even in the ladle that was continuously used, and conversely, even in the ladle that was not continuously used in the charge that had a long secondary refining time, As a result, the decrease in molten steel temperature was small. In addition, even if this secondary refining time was extended more than needed and the secondary refining time was lengthened, it turned out that the molten steel temperature fall rate is gradually saturated.
[0013]
From these facts, the inventors have stabilized the temperature of the inner wall refractory of the ladle until the heat transfer from the molten steel to the ladle reaches a steady state, that is, the secondary refining stage treatment by the ladle. It has been found that if the continuous casting time is continued for a long time, even if continuous casting is performed for a long time, a significant decrease in the molten steel temperature at the end of casting can be suppressed.
Since the time required for secondary refining in this invention is affected by the steel type, heat size, ladle dimension, ladle refractory type, casting temperature, casting time, and the content of the secondary refining treatment, Although it cannot be determined, it can be predicted in advance by heat transfer calculation based on the dimensions and thermal conductivity data of the materials that make up each facility, and operation data centered on the calculation prediction time should be accumulated. The specific time in actual operation can be determined.
[0014]
The ladle inner wall refractory is made up of refractory layers called work lining that are permanently stretched in order from the side closer to the iron skin of the ladle. Among these, the permanent tension is composed of a refractory material having a high heat insulating property, although the fire resistance and the erosion resistance are relatively low. On the other hand, the work lining has much higher fire resistance, erosion resistance, spalling resistance and the like than permanent tension, but has low heat insulation (that is, high thermal conductivity).
The heat that flows into the refractory through the work lining with high thermal conductivity from the molten steel in the ladle is blocked by the highly heat-insulating permanent tension layer and accumulated in the work lining. Then, due to the difference between the temperature of the boundary between the work lining and the permanent tension and the temperature of the iron skin, it gradually moves in the permanent tension layer and moves to the iron skin, and finally the atmosphere from the iron skin or vacuum degassing treatment. Dissipate into the tank. The temperature of the ladle iron skin does not change so much because it is cooled by the atmosphere or atmosphere. Therefore, the heat flux dissipated from the ladle through the iron shell is almost dependent on the temperature at the boundary between the permanent tension and the work lining. The fact that the temperature state in the ladle inner wall refractory reaches a steady state means that the above heat flux is stabilized, so whether or not the temperature in the ladle inner wall refractory has reached a steady state. It is most reasonable to grasp the temperature at the boundary between the permanent tension and the work lining.
Therefore, in the present invention, it is possible to grasp the thermal state of the ladle inner wall refractory, whether by heat transfer calculation, by actual measurement by a thermocouple, etc., the temperature at or near the boundary between the permanent tension and the work lining. Is practical and preferred.
[0015]
For example, as shown in FIG. 1, the secondary refining time during which the amount of heat released from the ladle becomes steady and the temperature of the refractory inside the ladle (the temperature at the boundary between permanent tension and work lining) is almost stabilized is 40 When the secondary refining is performed over this time, as shown in Fig. 2, the ladle is being cast both when it is used 3 times or more and less than 3 times as shown in Fig. 2. As a result, the temperature drop of 10 ° C. or less can be obtained.
[0016]
In the present invention, long-time continuous casting refers to the case of continuous casting for a time of 70 minutes / ch or more. In this case, as shown in FIG. 3, the secondary refining time is 20 minutes ( In the conventional example), while the continuous casting time: 70 minutes, the molten steel superheat degree starts to drop suddenly, whereas when continuously cast under the conditions suitable for the present invention (secondary refining time 60 minutes), Even at the end of casting, it was found that the degree of superheated molten steel was within the proper range of 35 to 55 ° C.
[0017]
【Example】
Hereinafter, experiments conducted for verifying the effects of the present invention will be described.
In this experiment, martensitic medium carbon stainless steel was used as a raw material, and this applied material was subjected to secondary refining, that is, vacuum degassing treatment by a VOD apparatus prior to continuous casting. This is an example in which the obtained 180 t / ch molten steel was continuously cast by a slab continuous casting machine.
The VOD treatment time is 20 to 90 minutes, 80 to 120 minutes of continuous casting time, and the molten steel superheat degree (° C) in the dundish is measured at regular intervals to reduce the molten steel temperature. Asked.
[0018]
As a result, FIG. 3 shows the temperature change during casting when the secondary refining time is 60 minutes as an example of the present invention and the secondary refining time is 20 minutes as a conventional example.
[0019]
As can be seen from this figure, when the casting time was 120 minutes, in the invention method, the molten steel temperature actually dropped to 40 ° C from 50 ° C when 40 minutes passed, but it was still within the proper range and cast. Was possible. However, with the conventional method, the casting temperature was 65 ° C. when 40 minutes passed, but the molten steel superheat decreased to 25 ° C. when 120 minutes passed, making it difficult to continue casting.
[0020]
【The invention's effect】
As described above, according to the present invention, even when continuous casting is performed for a long time, a special heat application facility such as a tundish molten steel heating device is unnecessary, and the molten steel temperature at the end of casting is not necessary. Therefore, it is not necessary to excessively raise the temperature at the beginning of casting (molten steel superheat degree), so that it is possible to solve variations in slab quality and operational problems. In addition, stable casting operation can be ensured in the entire casting section.
[Brief description of the drawings]
FIG. 1 is a graph showing the relationship between secondary refining time and ladle inner wall temperature.
FIG. 2 is a graph showing the influence of secondary refining time on the temperature drop of molten steel during continuous casting.
FIG. 3 is a graph showing the relationship between the change in molten steel temperature during casting and the degree of superheated molten steel in Examples.

Claims (1)

When performing continuous casting with a ladle 1 charge continuous casting time of about 70 minutes / ch or more, the ladle is processed in the secondary refining stage prior to this continuous casting. A pretreatment method for continuous casting over a long period of time, characterized in that it is carried out for about 40 minutes or more at which the temperature of the inner wall refractory is substantially stabilized.

Images